<div class="search_block_top">
  <form action="<?php echo site_url('search') ?>" method="get" id="form_search">
    <input type="text" placeholder="Input your keyword"
           value="<?php echo isset($keyword) ? $keyword : '' ?>" name="keyword" id="keyword" 
           class="search_query"/>
    <input type="submit" value="Search" class="submit"/>
  </form>
</div>